An evaluation of nebulized ammonium fluorescein as a laboratory aerosol

For many laboratory studies in aerosol research as well as for the calibration of size-relating aerosol measuring equipment, a suitable model aerosol should consist of spherical particles of uniform density at airborne concentrations which facilitate a convenient method of mass determination of an aerosol fraction sampled within a reasonable period of time. These and other requirements are met by aerosols generated with a nebulizer from dilute solutions of fluorescein in aqueous ammonia. The present study shows that, similar to procedures described for uranine, the original droplets emerging from the nebulizer dry up and form spherical particles of uniform density within a very short time after mixing with clean air of low humidity. In contrast to uranine aerosols, the ammonium fluorescein aerosols show no hygroscopicity. Their solubility in water is low, but by dissolving them in aqueous ammonia, a sensitive mass determination by fluorometric measurements under u.v. irradiation is feasible in the same way as for uranine. The density evaluation was made with the spiral centrifuge aerosol size spectrometer under operating conditions for high size resolution. The results indicated that the particle density of the new aerosol substance was independent of the particle size and had an average value of 1.58 g cm−3. This value is probably too high due to a slight systematic calibration error of the centrifuge. Conventional density measurements gave a value of 1.35 g cm−3. By employing the spiral centrifuge for the evaluation of the new aerosol, the study also reconfirmed the excellent optimal size resolution achievable with this instrument. A value of better than 2.5 per cent for sizes above 0.3 mm in aerodynamic diameter was found. For smaller sizes the study revealed a worsening of the resolution to 11 per cent at 0.08 mm, which is qualitatively in keeping with theoretical expectations.